Prediction and prevention of spontaneous preterm birth in specialised preterm prevention care

#Background

This section of Taonga Tuku Iho provides recommendations of practice on the identification and modification of risk for spontaneous preterm birth for those with a higher chance of spontaneous preterm birth and second trimester miscarriage due to indirectly modifiable risk factors. These risk factors require care for wāhine/people in addition to general population preterm prevention care, this is referred to as specialised preterm prevention care.

For the identification and modification of risk for spontaneous preterm birth and second trimester miscarriage relevant to pregnancy care for all wāhine/people, recommendations of practice for pregnancy care are provided in Prediction and prevention of spontaneous preterm birth in the general population. Recommendations of practice for the general population are also relevant to those receiving specialised preterm prevention care. Both sections should be considered for these wāhine/people.

For wāhine/people with signs or symptoms of preterm labour (threatened preterm labour) and/or preterm prelabour rupture of membranes (PPROM) (for the general and specialised preterm prevention populations), recommendations of practice on the prediction and prevention of spontaneous preterm birth in these clinical scenarios are provided in Preterm prelabour rupture of membranes (PPROM) and threatened and active preterm labour.

Spontaneous preterm birth may occur following the spontaneous onset of labour and/or PPROM <37⁺⁰ weeks gestation. The lower limit of preterm birth may be considered as birth from 20⁺⁰ weeks (as the timepoint requiring birth registration in Aotearoa New Zealand), although pēpi survival is only possible from 22-23 weeks. The mechanisms leading to spontaneous second trimester miscarriage (considered from 14⁺⁰ weeks gestation)¹ are similar to those causing spontaneous preterm birth. Therefore these two conditions (preterm birth and second trimester miscarriage) should be considered as the same pathophysiological entity with similar risk factors, risk modification, prediction and prevention. Due to the shared pathophysiological mechanisms and similar approaches to care, reference to spontaneous preterm birth in this section of Taonga Tuku Iho includes all births from 14⁺⁰ to 36⁺⁶ weeks.

There are numerous established risk factors for spontaneous preterm birth and second trimester miscarriage, and many of these risk factors can be identified prior to, or in early pregnancy. Once identified, pregnancy care can be adapted, aiming to limit the chance of the spontaneous onset of labour and/or PPROM. Preventative therapies are more likely to be successful than attempting to stop birth once PPROM and/or preterm labour has occurred.

Risk factors for spontaneous preterm birth may be considered to be:

• Directly modifiable – the risk factor can be removed or altered
• Indirectly modifiable – the risk factor cannot be taken away, but interventions are available to reduce risk
• Non-modifiable – the risk factor cannot be taken away or changed, but awareness may still be beneficial.

This section focuses primarily on wāhine/people with indirectly modifiable risk factors, and the specialised preterm prevention care that may limit their higher chance of spontaneous preterm birth and second trimester miscarriage. These risk factors are:

• Previous spontaneous preterm birth and/or PPROM ≤35⁺⁶ weeks
• Previous second trimester miscarriage (≥14⁺⁰ weeks)
• Congenital uterine and/or cervical anomaly
• Previous large loop excision of the transformation zone (LLETZ) with depth of excision ≥10mm or more than one procedure, without subsequent term birth
• Previous cone biopsy or trachelectomy, without subsequent term birth
• Previous caesarean section at advanced cervical dilatation and/or with extensive tear through cervix +/- vagina, without subsequent term birth
• Multiple (≥3) previous uterine instrumentation e.g. dilatation and curettage for termination of pregnancy and evacuation of retained products of conception, without subsequent term birth
• Connective tissue disorders e.g. Ehlers Danlos syndrome
• Previous pregnancy requiring ultrasound-indicated or rescue cerclage, or treatment with vaginal progesterone, due to a short cervix (without preterm birth).
• Short (≤25mm) or dilated/open cervix at ≤24⁺⁶ weeks.

The background for these risk factors is detailed in ‘Prediction and prevention of spontaneous preterm birth in the general population’.

Wāhine/people with these indirectly modifiable risk factors, may have additional risk factors for spontaneous preterm birth that may be present prior to, or in early pregnancy, and may also develop new risk factors during pregnancy. When multiple major risk factors co-exist, the chance of spontaneous preterm birth compared to those with a single risk factor is likely to be higher. Addressing directly modifiable risk factors (e.g. treatment of bacteruria, becoming smoke-free) in the specialised preterm prevention population, as for the general population, is important and likely to limit risk. The background for directly modifiable risk factors is detailed in ‘Prediction and prevention of spontaneous preterm birth in the general population’.

For this section on the prediction and prevention of spontaneous preterm birth in specialised preterm prevention care, we have provided significant background detail that can be accessed in the PDF below. This includes information on:

• Preterm prevention clinics and advisors
• Adjunct prediction tests (ultrasound cervical length assessment, vaginal biomarkers, combination risk assessment tools)*
• Interventions for prevention (cervical cerclage – transvaginal history-indicated, ultrasound-indicated and rescue, and transabdominal; progestogen therapy; cervical pessary).

*Adjunct prediction tests for spontaneous preterm birth include ultrasound cervical length assessment and vaginal biomarker tests. This section focuses on their use in the population with indirectly modifiable risk factors and being cared for through specialised preterm prevention clinics.

For the use of adjunct prediction tests for spontaneous preterm birth and second trimester miscarriage relevant to pregnancy care for all wāhine/people, recommendations of practice for pregnancy care are provided in Prediction and prevention of spontaneous preterm birth in the general population.

For wāhine/people with signs or symptoms of preterm labour (threatened preterm labour) and/or PPROM, recommendations of practice on the prediction and prevention of spontaneous preterm birth in these clinical scenarios, including adjunct prediction tests, are provided in Preterm prelabour rupture of membranes (PPROM) and threatened and active preterm labour.

#Recommendations and Practice

Guideline Recommendations (pink boxes) and Good Practice (yellow boxes) are provided as recommendations of practice. Comprehensive clinical oversight of māmā/person and pēpi wellbeing is required, and this may influence how these recommendations of practice are used.

Each recommendation of practice should be considered in consultation with wāhine/people and whanāu, including clear explanations to allow informed decision-making. Wāhine/people have the right to decline a recommendation of practice. In these circumstances, healthcare providers should follow their professional responsibilities for ongoing care. ^86-89

Cervical cerclage for the prevention of spontaneous preterm birth

Cervical cerclage may be placed by a transvaginal route – as a history- or ultrasound-indicated procedure or due to an open cervix, known as an rescue or emergency cervical cerclage. Less commonly, cervical cerclage may be placed by a transabdominal route – as a history-indicated procedure for those with the highest chance of spontaneous preterm birth based on their obstetric (and gynaecological) history.

Vaginal progesterone for the prevention of spontaneous preterm birth

Vaginal progesterone may be considered for wāhine/people with indirectly modifiable risk factors for spontaneous preterm birth receiving specialised preterm prevention care and/or those who develop a short cervix in pregnancy.

Cervical pessary for the prevention of spontaneous preterm birth

#Auditable standards

#Included guidelines

The search identified 15 guidelines relevant to the prediction and prevention of spontaneous preterm birth in a high-chance population that met criteria for high-quality and/or were recommended for use with modifications.^18,62-75 This included eight guidelines on the general management of preterm labour where they included content on risk factors and/or prediction and prevention of spontaneous preterm birth,^62-69 five on pregnancy screening,^18,70-73 one on guidelines for consultation,⁷⁴ and one on progesterone therapy.⁷⁵

Three of these guidelines were assessed to be high-quality in both Rigour of Development (score >60%) and in Overall Assessment (score >60%) and were recommended for use in clinical practice by the Review Panel.^69-71 Another two were assessed to be high-quality in Overall Assessment (score >60%) only and were recommended for use in clinical practice by the Review Panel, or for use with modifications.^18,72 The remaining 10 guidelines did not meet high-quality criteria, but were recommended for use with modifications by the Review Panel.^62-68,73-75

The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) guidelines on measurement of cervical length for the prediction of preterm birth⁷² and on progesterone use in the second and third trimester,⁷⁵ have been updated since the guideline search was undertaken (November 2021¹¹ and November 2023,⁷⁶ respectively). The New Zealand Referral Guidelines for Consultation with Obstetric and Related Medical Services,⁷⁴ were updated in 2026.⁷⁷ The International Society of Ultrasound in Obstetrics and Gynaecology (ISUOG) Practice Guidelines for performance of the routine mid-trimester fetal ultrasound scan⁷³ were updated in 2022.⁷⁸ The newer versions of these guidelines have been used. The 2019 Ministry of Health New Zealand Obstetric Ultrasound Guidelines were also updated in 2024; it is still pending publication in July 2026, however, the new recommendations (criteria for cervical length surveillance) have been used. The RANZCOG guideline on screening in early pregnancy for adverse perinatal outcomes⁷¹ was updated in 2024 to include preeclampsia only,⁷⁹ and hence was no longer relevant to this section.

To support the preparation of this section, additional guidelines were considered, including the International Federation of Gynecology and Obstetrics (FIGO) good practice recommendations on progestogens for prevention of preterm delivery,⁸⁰ FIGO good practice recommendations on cervical cerclage for prevention of preterm birth,⁸¹ FIGO good practice recommendations on the use of pessary for reducing the frequency and improving outcomes of preterm birth⁸² and ISUOG Practice Guidelines: role of ultrasound in the prediction of spontaneous preterm birth.⁴ The FIGO and ISUOG guidelines met the original search criteria as ‘international (defined as those developed for use in more than one country and with similar resources and health context to Aotearoa) and containing recommendations on prediction or prevention of preterm birth, preparation for preterm birth, or the acute management of preterm labour’,⁸³ but were published since the original guideline search. Due to the limited number of identified guidelines on this topic, these new guidelines have been considered, but they were not included in the Review Panel assessment.

Some additional guidelines that were also not identified by the original search were also considered in preparation for this section. These were: The NHS England Saving Babies' Lives Care Bundle Version 3;⁸⁴ the United Kingdom Preterm Birth Network ‘Reducing Preterm Birth’ Guidelines for Commissioners and Providers;⁶ and the Te Toka Tumai National Women’s Health Preterm Clinic Referral Guidelines.⁸⁵ These specifically informed recommendations on criteria for specialist high-chance spontaneous preterm birth care and management options, which were insufficient in the guidelines identified through the original search. These guidelines were also not included in the Review Panel assessment.

#Impact on equity

The vision of the Carosika Collaborative is ‘Equity in preterm birth outcomes will be achieved in Aotearoa by lowering preterm birth rates and optimising preterm birth care’ ; Taonga Tuku Iho is a major tool to support this. Equity has been prioritised throughout the development process of Taonga Tuku Iho and will drive its implementation and measurement of impact.

During the identification, evaluation and selection of the clinical practice guidelines that inform Taonga Tuku Iho, the Review Panel considered the potential impact on equity of recommendations within each guideline⁸³ and these are summarised here.

Review Panel guideline assessments identified that the recommendations in three of the 15 guidelines meeting criteria for consideration of inclusion in this guide had the potential to increase differences by equity factors,^66,69,75 and 12 of the 15 had the potential to reduce differences by equity factors.^62-71,73,75

Capacity for ultrasound services to deliver cervical length surveillance, especially via community providers, was noted by several Review Panel members. Issues related to access to cervical length screening for less affluent communities include the higher impact of part-payment charges that are often required to sustain community services, the shortage of sonographers (nationally but often more apparent in rural areas), and the skills and expertise of sonographers to perform high-quality cervical length assessments (often focused in major cities).

It was also noted that not all obstetric units have the capacity to manage a potential increase in workload if all wāhine/people with recognised indirectly modifiable risk factors for spontaneous preterm birth are referred for review. In addition, it was noted that secondary units that do not provide level 3 NICU services may have less expertise in managing pregnancy for those with a higher chance of early spontaneous preterm birth. It was also broadly noted that skill, policy and organisational barriers nationally may limit the ability to deliver equitable preterm birth prevention services.

For individual wāhine/people, late booking in pregnancy was identified as a significant barrier to be able to achieve appropriate and timely risk factor screening, surveillance and preventative treatment for spontaneous preterm birth. It was acknowledged that there are significant differences in first and early second trimester booking rates by ethnicity and region of residence.

#Current research

The ABOVE Trial is a UK-led randomised controlled trial to assess the optimal preventative management for preterm birth secondary to caesarean section damage. It includes wāhine/people who have had a second trimester miscarriage or birth <30 weeks after an in-labour caesarean section birth.⁹² Those who are included are randomised to receive a transabdominal cervical cerclage or transvaginal cervical cerclage, the primary outcome is birth <30 weeks. Recruitment is ongoing.

#Statement on rationale for any differing recommendations from the high-quality guidelines

The mechanisms leading to spontaneous second trimester miscarriage (often referred to as second trimester loss) are similar to those causing spontaneous preterm birth and hence should be considered and managed in a similar way. To date, second trimester loss is often considered only for miscarriage ≥16⁺⁰ weeks. However, the gestational age to define the lower limit of the second trimester is conventionally considered to be after completion of 12 weeks. For Taonga Tuku Iho, we have elected to use ≥14⁺⁰ weeks to define second trimester miscarriage and to identify wāhine/people for inclusion in our recommendations. This conflicts with New Zealand Obstetric Ultrasound Guidelines¹⁸ and Te Toka Tumai National Women’s Health Preterm Clinic Referral Guidelines,⁹³ that both refer to ≥16⁺⁰ weeks. We have adopted this gestation in accordance with the 2025 Mid-­trimester Pregnancy Loss Guideline Consensus Panel proposed global framework.¹

The criteria for referral for obstetric review due to indirectly modifiable risk factors have been developed using the New Zealand Obstetric Ultrasound Guidelines¹⁸ (which identify recognised risk factors that warrant referral for cervical length surveillance) and Te Toka Tumai National Women’s Health Preterm Clinic Referral Guidelines,⁹³ with some adaptations. A single previous uterine instrumentation is associated with an increased chance of spontaneous preterm birth, and this is further elevated after 2-3 procedures.⁹⁴ Te Toka Tumai National Women’s Health Preterm Clinic Referral Guidelines⁹³ includes ≥2 uterine instrumentations in its referral criteria. The currently published New Zealand Obstetric Ultrasound Guidelines¹⁸ does not include previous uterine instrumentation as a criteria (but it is included in the unpublished revised version). Taonga Tuku Iho has taken a pragmatic approach to use ≥3 uterine instrumentations in consideration of how the resources of specialist review and cervical length surveillance are utilised. The New Zealand Obstetric Ultrasound Guidelines¹⁸ include ‘previous Caesarean section at full cervical dilatation’ in referral criteria, in Taonga Tuku Iho this has been expanded to ‘Previous caesarean section at advanced cervical dilatation and/or with extensive tear through cervix +/- vagina without subsequent term birth’.

The ISUOG Practice Guidelines: role of ultrasound in the prediction of spontaneous preterm birth include a recommendation on cervical length measurement as the preferred method for screening for preterm birth in twins, with 25mm as the pragmatic cut-off at 18-24 weeks.⁴ This is identified as a ‘good practice point’ only, defined as a consensus-based best practice where formal evidence is lacking (lower ranked than grade D/level 4, which represents expert opinion or inconclusive evidence). For Taonga Tuku Iho, we have considered the implications of cervical length screening for all twin pregnancies, regardless of additional risk factors, and the impact on current resources. Without strong (any) evidence to support this practice, we have not included any recommendations or good practice for universal screening of cervical length, but have included guidance for those with a multiple pregnancy and additional risk factors for both screening and treatment. Aotearoa-specific guidance including the recent (2026) Wāhi Rua (New Zealand Maternal Fetal Medicine Network) Recommendation of Practice ‘Multiple Pregnancy’⁹⁵ and New Zealand Obstetric Ultrasound Guidelines¹⁸ do not include any guidance on universal cervical length screening for multiple pregnancy. The New Zealand Obstetric Ultrasound Guidelines include a reporting alert for an open cervix only.¹⁸

#References

References for this section include those used in Recommendations, About, and Specialised Preterm Prevention Care Background Summary sections.

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